During a flight mission, an aircraft pilot has to monitor his flight environment while carrying out the tasks of piloting, navigation and completing his mission. The task of the pilot becomes particularly complex during flight phases at low altitude to monitor the obstacles and the terrain and, in the case of a tactical mission, to monitor moving objects. It is therefore normal to assist the pilot in the execution of his piloting functions with augmented reality systems. Specifically, there is, for example, vision equipment that, in real time, adds to the visual perception of the pilot with synthetic information. Head-up display equipment is known that makes it possible to superpose piloting and navigation information on the real images of the visual field of the pilot. These systems consist in placing, in the visual field of the pilot, a transparent and semi-reflective optical strip on to which images collimated to infinity are projected. This transparent optical strip can be a mirror mounted on the cockpit of an aircraft or, if necessary, the visor of the pilot's helmet.
The use of the vision equipment inside a cockpit is problematical for several reasons. First, this equipment is used during day and night missions. The result of this is therefore that the level of luminosity varies greatly and even regularly when the weather is cloudy. The visibility of the information shown in bright weather requires an image of strong luminosity and for that, in the case of helmet vision equipment, a solar visor is usually used for uniform transmission of approximately fifteen percent. There are also visors that can be interchanged with a higher transmission suitable for bad conditions of daytime visibility. However, when the pilot lowers his head to see the information on the instrument panel, he then has to lift the solar visor so as not to degrade the vision of the multifunctional screens. Similarly, during a tactical flight, the pilot has to look towards the bright sky, and then towards the ground where zones are in shade from the sun. He then has to manipulate his visor frequently and rapidly.
There are also vision systems coupled with thermal imaging cameras that superpose a video image on the image of the landscape. These systems make it possible to augment the vision of the terrain in the event of landing in bad visibility conditions due to fog, dust, etc. The visibility of this daytime image requires a strong contrast which the lowering of a solar visor does not process correctly because its presence also attenuates the residual visibility of the landscape.
To assist the pilot, night-vision systems integrated with a helmet visor with on-visor projection are also known. The night-time image is superposed on the landscape image seen through the visor. In such systems, the night-image lens is offset from the eyes in order to free up the view of the landscape. The result of this is a stereoscopic vision different from natural stereoscopic vision. At a short distance, this stereoscopic vision results in the night image not being superposed with the natural image resulting in a triple vision (real image, left night image and right night image or night image, right real image and left real image), the merging or convergence usually taking place on the brightest image. These triple images can disrupt the pilot notably when the scene comprises luminous points such as for example in an urban environment or on the landing runway. In these conditions, it is worthwhile to be able to mask the direct vision and give preference to night vision. One solution is for example the use of an additional partly opaque visor. The opaque portion of the additional visor is placed in front of the binocular image in order to augment its contrast and leave the lateral visual field free for retaining a good perception of the environment. This visor has to be removed rapidly for reasons of safety. However, this solution is not satisfactory for the same reasons mentioned above.